Characterization and Reduction of Audible Magnetic Noise Due to PWM Supply in Induction Machines

This paper derives the analytical characterization of the Maxwell radial vibrations due to pulsewidth modulation (PWM) supply in induction machines and, particularly, in traction motors supplied with an asynchronous switching frequency. The number of nodes and the velocity of these particular force waves are experimentally validated by visualizing some operational deflection shapes of the stator. It is shown that according to the switching frequency, these forces can be responsible for high magnetic-noise levels during starting and braking. A simple rule to avoid PWM noise is then proposed and applied to an industrial traction motor. Experimental results show that the choice of the switching frequency can have a 15-dB impact on the sound power level emitted by the motor during starting and that a lower switching frequency can sometimes lead to lower magnetic noise. In agreement with analytical predictions, the new proposed switching frequency that avoids resonances between PWM exciting forces and corresponding stator modes reduces the magnetic noise of 5 dB during starting.